Apply Igor's fix:

Fixed a failure of the merge_innodb test on windows.


storage/myisammrg/ha_myisammrg.cc:
  Fixed a failure of the merge_innodb test on windows.

storage/myisammrg/ha_myisammrg.cc

@@ -309,7 +309,7 @@ static MI_INFO *myisammrg_attach_children_callback(void *callback_param)
   TABLE         *parent;
   TABLE         *child;
   TABLE_LIST    *child_l;
-  MI_INFO       *myisam;
+  MI_INFO       *myisam= NULL;
   DBUG_ENTER("myisammrg_attach_children_callback");
 
   my_errno= 0;